Electrical resistor device



April 24, 1956 E. F. ZOLTANSKI 2,743,340

ELECTRICAL RESISTOR DEVICE Filed May 28, 1952 INVENTOR. EDWARD F. ZOLTANSKI ATTORNEY.

United States Patent 0 2,743,340 ELECTRICAL RESISTOR DEVICE Edward F. Zoltanski, Trenton, N. J., assignor to Minneapolis-Honeywell Regulator Company, Minneapoiis, Minn., a corporation of Delaware Application May 28, 1952, Serial No. 296,601 1 Claim. (Cl. fill-55) The general object of the present invention is to provide a new and improved variable electrical resistor. More particularly, the present invention is concerned with an electrical resistance element wherein an electrically conducting fluid is arranged to short out an electrically conducting coating on the inner surface of a hollow insulating element.

Heretofore it has been common practice to build certain types of variable resistance devices by placing a relatively fine resistance wire in the hollow portion of a capillary tubing and variably shorting out the wire by means of a mercury column or other electrically short circuiting fluid. Such a device is shown and described in the patent of William H. Bristol, No. 776,491, issued December 6, 1904. Such devices as disclosed in the Bristol patent have been in use up to the present time and have been known to have inherent faults which can not be eliminated as long as a resistance wire must be placed in the device. Among the difliculties of these prior devices is the problem of obtaining high resistance changes with relatively low movement or displacement of the shorting fluid. Another difiiculty has been that of attempting to center the resistance wire in the center of the tube so as to have the shorting fluid level changes efiect uniform changes in resistance of the element.

Another difficulty with devices incorporating a metallic resistance wire within the tube is the resistance wire, and its impurities, often acts to contaminate the mercury or shorting fluid so that long time uniform results can not be attained. Another difficulty lies in the fact that shorting fluids such as mercury have a different wetting action on the resistance wire than on the glass and as a result there is a considerable hysteresis error introduced into such devices and yield poor reproducible characteristics. This is also evidenced by the obstructionist efiects of a wire in a small capillary. Another difficulty of these prior devices, where an attempt was made to utilize a very fine wire as a resistance wire, is that such devices are diflicult to calibrate, presumably due to the inability of the mercury to make reproducible contact with such a fine wire. Further, with such fine wire, it is impossible to apply a readily available operating voltage to the wire so that it may be utilized in an electrical circuit.

It is accordingly an object of the present invention to provide a new and improved variable resistance element which will overcome the above enumerated deficiencies of the prior art devices.

Another object of the present invention is to provide a variable resistance element comprising an electrically conductive coating on a ceramic material wherein said coating is variably shorted out by suitable highly conductive fluid.

Still another object of the present invention is to provide a transducer element of the electrical type having a hollow ceramic body with an electrically conductive coating on the inner surface thereof with a highly conductive fluid for variably shorting out said coating between a pair of electrical contacts which are in contact with the electrical coating.

A further object of the present invention is to provide a new and improved variable resistance element comprising a hollow non-porous ceramic material having an electrically conductive coating on the inner surface thereof wherein the coating is formed of a metallic oxide with an electrically conducting fluid for shorting the coating and varying the resistance between a pair of electrical contacts engaging the coating.

The various features of novelty which characterize the invention are pointed out with particularity in the claim annexed hereto and forming a part of the specification. For a better understanding of the invention, however, its advantages, and the specific objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described preferred embodiments of the invention.

Of the drawings:

Figure 1 is a schematic showing of one form that the invention may assume and how it may be utilized with a suitable electrical signal utilization circuit;

Figure 2 is a diagrammatic showing of the present invention as applied to a flow meter; and

Figure 3 is a diagrammatic showing of one form the invention may assume when applied to a pressure to resistance transducer application.

Referring now to Figure 1, the numeral 10 represents a thermometer which includes a reservoir or bulb 11 which is filled with a low resistance or highly conducting fluid which, in this particular embodiment, should also have a high thermal coeflicient of expansion. Such a fluid may be, for example, mercury.

Mounted on top of the reservoir or bulb 11 is a capillary tube 13 which is shown in exaggerated fashion in order to emphasize the details. This capillary 13 is preferably made of Pyrex glass and is arranged to have a relatively small inside diameter which may be on the order of one millimeter. The inside of this capillary 13 has an electrically conductive coating on the surface thereof. This coating is formed in a continuous layer throughout the length of the capillary 13 on the inner surface thereof and has a relatively high resistance. This conductive coating or film may be in the form of a metallic oxide such as is disclosed in the John M. Mochel patent, 2,564,706, issued August 21, 1951. This electrically conductive coating has a relatively high resistance compared to the electrical resistance of the pure metal which is used in the metallic oxide of the coating.

Making electrical contact with the fluid 12 is a suitable electrically conducting electrode 15 which is selected so as not to contaminate the fluid 12. When the fluid 12 is extending into the capillary 13 it will be seen that the electrode 15 is coupled to the conductive coating 14 by the fluid. Inserted through the upper end of the capillary and engaging the electrically conductive coating at the upper end is a further electrode 16.

This variable resistance thermometer may be connected as the leg of a Wheatstone bridge 20 which includes a source of power 21, a fixed resistor 22, and a rebalancing potentiometer 23. The output of the bridge 20 may be connected to a suitable recording and controlling apparatus 24 which is arranged to reversibly drive a control motor 25, the latter of which is arranged to adjust the rebalancing potentiometer 23 and the bridge 20. This recording and controlling apparatus 24 may be of the type disclosed in the Walter P. Wills patent, 2,423,540, issued July 8, 1947.

In considering the operation of the apparatus shown in Fig. 1, it will be understood that temperature changes around the thermometer bulb 11 will cause the fluid therein to rise and fall in accordance with the changes.

These changes in turn will serve to short out the conductive coating in accordance with the height of the fluid in the capillary 13. As the fluid column rises, the effective resistance between the contactors 15 and 16 is efiective'ly decreased and this decrease in resistance will be reflected through the .Wheatstone bridge 2%) into the 'apparatus 24 which will be effective to record the resistance change and also to readjust the balancing potentiomtater-23.

border that the variable resistance thermometer 10 have a minimum of hysteresis between rising and falling temperatures, it is essential that extreme cleanliness be practiced in the assembling of the thermometer. This means that the inner surface of the capillary must be completely free'of impurities and that the mercury, or other fluid, used in the apparatus is also free of impurities where those impurities might cause separation of the fluid in the capillary or undue meniscus changes with the rise and fall of the fluid in the capillary.

' By proper regulation of the coating process it is possible to vary the resistance of the coating per square. It has been found possible for the one millimeter inside diameter tube to have an overall resistance of between one thousand to ten thousand ohms per foot length of the capillary tube.

It can thus be seen that relatively large resistance changes can be obtained with relatively small variations in the position of the shorting fluid column in the capillary. 'Also, by having both a transparent capillary 13 and utilizing an electrically conductive coating which is also transparent it is possible to visually observe the temperature as well as obtain an electrical resistance which varies with temperature.

Referring to Figure 2, there is shown one form the invention may assume in a flow measuring apparatus.

Here a U tube 39. is disposed withthe ends thereof at 31 and 32. This U tube may have mercury or other suitable highly conducting material in the lower portion of the U tube. The inner portion of the U tube may have an 'electricallyconductive coating on all or on a portion thereofas at 33. A pair of electrical contacts 34 and 35 are ,used to contact the electrically conducting coating 33.

It will be obvious that variations in the flow of the fluid through the orifice 29 will be effective to vary the level of the fluid in the U tube so as to vary the electrical resistance between the contacts 34 and 35. It will be obvious that these resistance changes will be proportional to changes in flow of fluid through the orifice 29.

Referring to Figure 3 there is shown a further embodiment of the invention as applied to a pressure to resistance transducer. Here there is provided a collapsible reservoir 40 containing a suitably conductive fluid 41. This fluid is arranged to rise and fall in a suitable capillary 42 having an electrically conductive coating thereon at 43. A pair of electrical contacts 44 and 45 are provided for contacting the surface of the conductive coating 43. This entire element may be supported in a chamber 46 wherein the pressure is to be measured by a supporting and sealing element 47 which may be a neoprene stopper with a center hole therein.

The apparatus of Figure 3 produces changes in the electrical resistance between the contacts 44 and 45 by having the fluid rise and fall in the capillary tube 42. This rise and fall will be proportional to the increases and decreases of the pressure applied to the flexible chamber 43 which in turn forces the liquid up or down in the tube 42.

While, in accordance With the provisions-of the statutes, 1 have illustrated and described the best forms of embodiment of my invention known to me, it will be apparent to those skilled in the art that changes may be made in the forms of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claim, and that in some cases certain features of the invention may be used to advantagewithout a corresponding use of other features.

Having now described the invention, what I claim as new and desire to secure by Letters Patent, is:

An electrical variable sensing device comprising, a hollow ceramic body having a continuous layer of electrically conducting but resistive coating over a portion of the'surface defining the hollow portion of said body, said coating being formed of a composition of elements, a fluid having a negligible electrical resistance disposed in cooperative relation to said coating to electrically shunt said coating when in contacting relation therewith,

a pair of electrical contacts arranged to engage said electrical coating at spaced points with one of said pair continuously engaging said fluid, and means for varying the relation of said fluid with respect to said coating in accordance with changes in said variable.

References Cited in the file of this patent UNITED STATES PATENTS 2,119,680 Long June 7, 1938 2,429,420 McMaster Oct. 21, 1947 2,440,691 Jira May 4, 1948 2,566,369 Putman Sept. 4, 1951 

